British Prime Minister Theresa May was last night under pressure to retaliate against Russia for its "brazen act of war" in using nerve gas on the streets of Britain.

Gavin Williamson, the UK defence secretary, said the government was being "pushed around" by Russia and "we have to change the way we deal with it" amid calls for fresh sanctions against Moscow and the expulsion of Russian diplomats.

Boris Johnson, the foreign secretary, had already threatened sanctions if Russia was found to be behind the attack that left Sergei Skripal, a former double agent, and his daughter Yulia in comas and Nick Bailey, a police sergeant, in intensive care.

Yesterday Mr Johnson repeated his insistence that Britain would respond "robustly" against any country found to be responsible.

Consequences

Downing Street said it could not make a diplomatic move until it was proved who was behind the attempted murders.

Facebook photo of Yulia Skripal

A Whitehall source said: "There is a process to go through. These things have to be done in the right order."

Amber Rudd, the home secretary, told the House of Commons there would be consequences for the perpetrators, who would be brought to justice "whoever they are and wherever they may be".

Tom Tugendhat, Tory chairman of the foreign affairs select committee, made it clear to Mrs May that "the first duty of government is to protect the British people - I think using nerve agents on British streets really does demand a response".

Mr Williamson, who has claimed in the past that Russia was ready to kill "thousands and thousands" of British people, said yesterday it was becoming an "ever-greater threat", particularly to its closest neighbours.

He said: "Russia's being assertive, Russia's being more aggressive, and we have to change the way we deal with it because we can't be in a situation in these areas of conflict where we are being pushed around by another nation."

Senior politician Edward Leigh said the circumstantial evidence against Russia was "very strong" and told the Commons: "If Russia is behind this, this is a brazen act of war, of humiliating our country."

Ms Rudd said: "There will come a time for attribution and there will be, then, consequences and there will be further information that follows."

She said the investigation must be based on "facts, not rumour", but added: "The use of a nerve agent on UK soil is a brazen and reckless act.

"This was attempted murder in the most cruel and public way.

"People are right to want to know who to hold to account.

"We are committed to doing all we can to bring the perpetrators to justice - whoever they are and wherever they may be."

Mr Tugendhat, a former British Army officer, said it now seemed likely that Mr Skripal's wife Lyudmila and son Alexander had both been murdered.

The circumstances of their deaths in 2012 and 2017 have been disputed, with some reports claiming they died in car crashes, others stating health reasons.

Pointing to threats made against Col Skripal and the similarity of the attack to a fatal one on Alexander Litvinenko, the Russian dissident, Mr Tugendhat said: "We are beginning to see not only a very strong pattern but a very strong centre to that pattern - and that centre appears very strongly to be the Kremlin."

Cemetery

Mr Johnson said: "I think we have to wait for the outcome. But if, and it's still a big if, it turns out this is the agency of another state attempting to kill people on UK soil, then of course the UK will respond robustly."

Meanwhile, part of the cemetery in Salisbury where Mr Skripal's wife is buried has been cordoned off by police. The cordon around the spy's house, some five kilometres away, has also been widened.

Ms Skripal was understood to have died of cancer in 2012, aged 59.

Alexander died in St Petersburg last July and is commemorated with a plaque at the cemetery.